The present invention relates to a four-cycle engine which has an air-intake valve and exhaust valve.
In a conventional four-cycle engine, a cam shaft is provided separately from a crank shaft. The speed of rotation of this cam shaft is reduced to 1/2 of that of the crank shaft by means of a reduction gear comprised of, for example, gears, sprockets or the like, thereby causing a lifting of the air-intake valve and exhaust valve through the use of a cam provided on the cam shaft. This lifting system of prior art, which uses such a cam shaft, has the following drawbacks. (1) It is necessary to equip the valve gear with the above-mentioned cam shaft, which results in an increase in the manufacturing cost involved. (2) A large amount of time is required for machining the gears and sprockets used for speed reduction. (3) Mechanical noises are great in intensity. (4) A chamber for reception of the crank need be large with the result that it is difficult to miniaturize the section associated with the valve gear. (5) Since the air-intake valve and exhaust valve are disposed closely to each other, there is a likelihood that gas exchange becomes incomplete. And, (6 ) Since the air-intake valve and exhaust valve are close to each other, the cylinder undergoes a high thermal distortion.